In order to abate emission of carbon dioxide gas from automobiles, a reduction in weight of automobile vehicle bodies has been promoted by using high-strength steel sheets. Further, in order also to secure the safety of a passenger, a high-strength steel sheet has been increasingly used for an automobile vehicle body in addition to a soft steel sheet. In order to further promote the reduction in weight of automobile vehicle bodies from now on, a usage strength level of the high-strength steel sheet has to be increased more than conventionally, and in order to use the high-strength steel sheet for an underbody part, for example, local deformability for burring has to be improved.
However, when a steel sheet is increased in strength in general, formability decreases, and as shown in Non-Patent Document 1, uniform elongation important for drawing and bulging decreases. In contrast to this, as shown in Non-Patent Document 2, there is disclosed a method of securing uniform elongation even with the same strength by making a metal structure of a steel sheet complex.
Meanwhile, there is also disclosed a metal structure control method of a steel sheet that improves local ductility typified by bending, hole expanding, and burring. Non-Patent Document 3 discloses that controlling inclusions, making a structure uniform, and further decreasing hardness difference between structures are effective for bendability and hole expanding.
This is to improve hole expandability by making a structure uniform by structure control, but in order to make a structure uniform, as shown in Non-Patent Document 4, a heat treatment from an austenite single phase becomes a basis of manufacture. Further, in order to achieve strength and ductility, Non-Patent Document 4 also discloses a technique in which metal structure control is performed by cooling control after hot rolling, precipitates are controlled, and a transformation structure is controlled, thereby obtaining appropriate fractions of ferrite and bainite.
Meanwhile, Patent Document 1 discloses a method in which a finishing temperature of hot rolling, a reduction ratio and a temperature range of finish rolling are controlled, recrystallization of austenite is promoted, development of a rolled texture is suppressed, and crystal orientations are randomized, thereby improving strength, ductility, and hole expandability.